The present invention relates to printing or marking apparatus, and is particularly concerned with heated printing or marking apparatus in which at least a portion of the heating structure can be exchanged, removed or modified in order to control the temperature of the printing or marking element.
Product marking or coding operations are often carried out using thermoplastic ink compositions of the type which are solid at normal room temperatures, and which are rendered liquid or flowable at elevated temperatures. The ink composition must be kept heated while printing, but quickly cools and solidifies after it is applied so that the printed surface can be handled immediately without smearing the printed image. The ink supply is normally provided in the form of a porous ink roll that is impregnated with a thermoplastic ink composition of the type described, and these rolls are convenient to handle and store when the ink is in the solid state. Common types of product marking operations in which ink compositions of this type are used include date coding, lot or batch numbering, and the like. In general, the object is to print some type of variable information on products or product wrappers, using a very simple type of marking or coding apparatus which can be installed on an existing conveyor system.
U.S. Pat. No. 4,559,872, issued to Andrew G. Perra, Jr. on Dec. 24, 1985 and assigned to the assignee of the present invention, discloses a printing apparatus which utilizes a hot-melt ink composition for product marking or coding operations. The disclosed apparatus comprises a porous inking roll of the type described previously, an adjacent printing roll carrying one or more printing elements to which the ink is applied, and a stationary heater block containing a number of electrical heating elements for heating the inking roll and printing elements. The heater block is generally in the form of an inverted "U", with its closed end surrounding the inking roll and its open end partially surrounding the printing member. By heating both the inking roll and printing elements from the stationary heater block, a very simple and reliable printing apparatus is obtained. Prior arrangements employed ring and brush assemblies to conduct electrical current to heating elements within the rotating printing roll or type holder, and this led to undesirable complexity and component failure.
Although the apparatus described in U.S. Pat. No. 4,559,872 has proved useful in product marking and coding applications, there are situations in which it is desirable to modify the relative temperatures of the inking roll and printing elements. For example, certain types of packaging films with low softening points may require that the printing elements be maintained at a lower temperature than the inking roll, in order to prevent melting or deformation of the film. Conversely, in applications where heat-sensitive films are not being printed, it may be desirable to maintain the inking roll and printing elements at similar temperatures.
Various measures have been employed in the design of the printing apparatus to vary the relative temperatures of the inking roll and printing elements. One approach that has been employed with some success is to modify the basic heater block design to add a cylindrical extension, referred to as a heat pipe, which extends into the interior of the hollow printing roll to provide supplemental heating to the printing elements. The problem with this expedient, however, is that it is of a permanent nature and does not allow the end user to exercise any significant degree of control over the inking roll and printing element temperatures after the printing apparatus has been put into use.